The influence of oxygen on the wear of WC-Co sliding against copper

Abstract

In the zipper manufacturing industry, cemented carbide (WC-Co) tools are typically used in shearing operations of pre-formed Cu-Zn alloy wire (15 wt% Zn) to form zipper elements. Despite its superior hardness, the WC-Co becomes gradually worn when shearing the alloy. Paradoxically, the WC and Co binder, despite their vastly different hardness, become worn at the same rate and to an even level, resulting in a smooth tool surface. High-resolution analysis of worn tools from zipper production and simplified tribological tests mimicking the sliding involved have led to several insights about the wear. The wear likely occurs through oxidative mechanisms and decreases with increasing Zn content in the alloy since Zn consumes available oxygen in the contact. In the present study, the oxidative nature of the wear is further analysed. The wear of WC-Co is investigated when sliding against pure Cu in different atmospheres, using a sliding test rig equipped with a controlled atmospheric chamber. The chamber is flushed with nitrogen gas to enable wear testing in an oxygen deficient environment. In addition, tests in flowing and stagnant air are performed to study the effects of an air flow on the wear. The wear is analysed using scanning electron microscopy, energy dispersive x-ray spectroscopy and optical surface profilometry. The visual appearances and chemical compositions of the wear surfaces were very similar after sliding against pure Cu in the different atmospheres, indicating that the same wear mechanisms had been active. In contrast, the wear rate differed significantly between the different atmospheres, being lowest in the oxygen deficient atmosphere and highest in flowing air. This indicates that the wear is oxidative, and that sliding in an oxygen deficient atmosphere has a similar effect of reducing wear as the addition of Zn has in Cu-Zn alloys. • Crossed cylinders sliding tests with WC-15Co and pure Cu in different atmospheres • Test is enclosed in a small volume with a flow of nitrogen or air, or stagnant air. • Resulting wear surfaces of WC-Co are all even and smooth, and wear is shallow. • Lowest wear rate is observed in nitrogen, hence the oxygen deficient atmosphere. • The WC-Co degrades through oxidative mechanisms occurring at a nanometre scale.

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